I'm dumb so I can't explain it that well but here goes the difference, according to google:

Fog is denser and thicker than mist, reducing visibility to less than one kilometer (0.6 miles), while mist can reduce visibility to between 1 and 2 kilometers (0.6 - 1.2 miles). Fog is formed when any cloud type makes contact with the ground, while mist can form a fairly thin, greyish veil.